In a magnetic disk drive such as an external storage device of a computer, a magnetic head follows any target track on a rotating magnetic disk surface, and data is recorded on the track for reproduction. This requires a control application to make the magnetic head lie on the target track accurately. Recently, there are tendencies to reduce the track width for the purpose of increasing the storage capacity of the magnetic disk drive, thereby requiring to increase, to a further degree, the accuracy and speed for positioning.
The reasons of causing the reduction of the positioning accuracy include the vibration of a position signal that is synchronous with the rotation of the disk, and the vibration of a mechanism system. These vibrations are observed in a specific frequency being an integral multiple of the rotation frequency of the disk, a natural frequency of the mechanism system, or others. For such vibrations, it is considered effective to design a control system using a digital filter (resonance filter) having a resonance point at the frequency. Such a resonance filter does not perform a calculation operation during a seek operation for the aim of preventing a transient response from occurring, and the resonance filter starts operation after a head reaches close to a target track. It thus takes time to suppress any disturbance after the head reaches close to the target track.
As a technology for addressing this issue, U.S. Pat. No. 5,608,586 describes a technique of making a resonance filter follow, at high speed, any vibration at a specific frequency with no transient response. This previous technology applies a method in which the resonance filter is operated for continuous calculations during a seek operation with an input of 0 to the resonance filter, and the resonance characteristics implemented by the resonance filter are left for free vibration. The reference “Compensation for Repeatable Tracking Errors in Hard Drives Using Discrete-Time Repetitive Controllers” IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 6, NO. 2, JUNE 2001, p. 132-136) describes a method of repeatedly increasing the learning speed of a control system. With this previous technology, for any vibration synchronous with the rotation, two types of filters are switched from one to the other to shorten the learning time.
As described in U.S. Pat. No. 5,608,586, with such a technique of setting 0 to an input for the resonance filter during a seek operation, an internal variable of the resonance filter is required to follow the vibration characteristics before the seek operation. Therefore, when the vibration characteristics with any target frequency show a gain difference or a phase difference before and after the seek operation, the positioning accuracy may be reduced. Especially with a seek operation attended with a head change, a long-distance seek operation, or a periodic disturbance at high frequencies, vibration components before and after the seek operation may have no correlation. In this sense, under such requirements, this technique is difficult for application use.
With the method of “Compensation for Repeatable Tracking Errors in Hard Drives Using Discrete-Time Repetitive Controllers” IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 6, NO. 2, June 2001, p. 132-136, because every vibration component synchronous with the rotation is subjected to learning, there is a problem of being difficult to shorten the learning time only for a specific frequency. With the actual magnetic disk drive, vibration synchronous with the rotation is not generated uniformly but mainly at a specific frequency. In this sense, the application limitation to a specific frequency is a great restriction.